The quality of a product obtained in an injection molding machine for producing a product by performing a plasticizing/measurement process (hereinafter referred to as a measurement process) and an injection/pressure maintenance process (hereinafter referred to as an injection process) is dependent on the injection conditions, and it is therefore important that the injection conditions be adjusted so that a satisfactory product is obtained.
The injection conditions include V-P switching control for switching the screw control from speed control to pressure control, and a technique for obtaining a satisfactory product by correcting the V-P switching position is disclosed in JP 2005-335078 A.
The injection conditions during the abovementioned injection molding will be described hereinafter with reference to the graph of FIG. 18 hereof.
In FIG. 18, the horizontal axis indicates the screw position, and the vertical axis indicates the injection pressure or the injection rate.
The reference symbol Ps indicates the curve of the injection pressure when a satisfactory product is obtained, P indicates the curve of the injection pressure in a certain shot, and V1 through V3 indicate lines that show the injection rate.
The screw position when the pressure curve Ps changes to the screw position correction detection pressure Pm is indicated by “A,” the screw position when the pressure curve P in a certain shot changes to the screw position correction detection pressure Pm is indicated by “a,” and a deviation of α (wherein α=A−a) exists between the two positions.
At this time, the speed of the screw switches from V1 to V2 (or V2 to V3) in the b positions (2 positions) that are offset by an amount equal to α from the B positions (two positions). Speed control is then switched to pressure maintenance control in the c position that is offset by an amount equal to α from the C position.
Since the pressure curve P of a certain shot can be made to imitate the pressure curve P that produces a satisfactory product by correcting the screw position by an amount equal to α, a satisfactory product can also be obtained in the certain shot.
In FIG. 18, the sequence of operations whereby the deviation α is detected after St, which is the starting point of the injection process, and the speed switching point or the speed-pressure maintenance switching point is corrected by an amount equal to the deviation α is performed within the injection process.
In recent high-speed molding methods, approximately 0.1 second elapses from the start St of the injection process to the start c of pressure maintenance control. The sequence of operations described above is difficult to perform in such a short time, and even if these operations could be performed, a high-sensitivity sensor or a control unit capable of performing high-speed computation is necessary, and device costs increase.
In JP 2005-335078 A, only the relationship between the injection pressure and the screw is managed. In this configuration, a satisfactory product becomes difficult to obtain when the temperature of the heating cylinder, the cycle time, and other factors fluctuate. Specifically, multiple fluctuating factors must be considered in order to obtain a satisfactory product.